The invention relates to an overlay, superimposing, or superposition steering system and method for motor vehicles (hereinafter referred to as an “overlay system” and an “overlay method”).
DE 198 59 806 discloses a steering system for motor vehicles that includes a driver-adjusted mechanism for the steering angle using a steering handle, and a control and regulating apparatus that activates the steered vehicle wheels using a motor-driven adjusting assembly. For emergencies, an automatically activated forced coupling is used between the steering handle and the steered vehicle wheels. This forced coupling has a centering mode that can be activated allowing for the adjustment of the steering wheel, while the forced coupling is at least partially switched off, until a position is reached in which the steering wheel is in a normal position when the steered vehicle wheels are in a straight position.
The disadvantage of this steering system is that the switching from an electronic steering (steer-by-wire) to a mechanical forced coupling between the steering handle and the vehicle wheels is a potential source for errors.
Therefore, it is an object of the present invention to provide a reliable centering system for an overlay steering system of a motor vehicle. The overlay steering system has a mechanical connection between the steering handle and the vehicle wheels.
This object is achieved with an overlay steering system for motor vehicles described hereinafter.
An overlay steering system of the present invention can generate, in addition to the driver steering angle, computer-defined steering angle components by way of an overlay actuator. This way, it is possible to change the path relationship or angle relationship between a steering apparatus and a steering angle of the wheels as a function of different values. At the same time, a permanent mechanical connection between the steering apparatus, preferably a steering wheel, and the steered wheels, remains in effect. At the first start-up of the system following a disruption or a renewed start-up following steering action with a passively shifted steering system, the relationship between the position of the steering apparatus and the position of the steered wheels may deviate in relation to the normal operating state. A deviation can also occur if an external disruption is compensated for using a controlled steering angle component while the vehicle is brought to a stand-still.
The method according to the invention eliminates the offset between the position of the steering apparatus and the steered wheels a manner that is not noticeable to the driver, non-disruptive and safe. This is accomplished by utilizing the overlay actuator and the computer of the active steering system to center the steering.
The overlay steering system according to the invention for a motor vehicle may include (1) a mechanical steering apparatus for the transfer of a steering torsional moment with a predetermined transfer ratio to at least one steered wheel, (2) an overlay actuator between the steering apparatus and the at least one steered wheel for the overlay of the steering torsional moment, (3) a steering angle sensor connected to the steering apparatus for generating a steering angle signal that corresponds to an angle of the steering apparatus, (4) a wheel angle sensor connected to the at least one steered wheel for generating a wheel angle signal corresponding to an angle of the at least one steered wheel, (5) a comparison device for comparing the steering angle signal and the wheel angle signal and for generating a comparison signal based on the steering angle and the wheel angle, and (6) an electromagnetic converter for generating the overlay torsional moment to reduce the deviation if the comparison signal indicates a deviation from the predetermined relation between the steering angle and the wheel angle of the at least one steered wheel.
The overlay torsional moment may be generated when the vehicle is standing still. In this case, the position of the mechanical steering apparatus is adjusted until the deviation from the predetermined relation is eliminated.
During travel the overlay torsional moment is generated in such a way that it has the same direction as the steering torsional moment, causing the actual transfer ratio to be larger than the predetermined transfer ratio.
Furthermore, the overlay torsional moment can be determined during travel based on the output signal of a speed sensor.
Other parameters for determining the overlay torsional moment during travel may be the steering torsional moment and the steering angle.
In another preferred embodiment, an acoustic and/or optical signal is emitted if the comparison signal indicates a deviation from the predetermined relation between the steering angle and the wheel angle of the at least one steered wheel, or if the overlay torsional moment is larger than a predetermined value.
In another preferred embodiment of the invention, the overlay torsional moment is always smaller than a predetermined value.
In another preferred embodied example of the invention, the overlay torsional moment is generated only when the vehicle is stopped.
The overlay steering system according to the invention causes the kinematic connection between the steering wheel and the front wheels to become separated, but not the connection in terms of moment between the two. This has the advantage that the driver continues to sense a “steering feel”, i.e. the effect of uneven pavement etc. from the steered wheels to the steering wheel via the steering torsional moment. Moreover, the steering torsional moment of the driver continues to be utilized for adjusting the wheels.
Another advantage of the invention is that the centering of the steering apparatus is adjusted automatically, except in unusual operating states, following delivery to the customer and as early as when the vehicle leaves the assembly line. In addition, all deviations of the steering wheel angle from the normal position are eliminated upon start-up of the vehicle or during travel. A repair shop visit is no longer necessary.
Other characteristics and examples of the invention can be derived from the following description of a preferred embodiment referring to the drawings.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.